The Taxon-Level Problem in the Evolution of Mammalian Brain Size: Facts and Artifacts

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Abstract
The allometric exponent relating brain to body size across diverse selections of mammals is about 0.75, when calculated as the slope of the line of best fit from logarithmically transformed data. However, the slope is known to change with the taxonomic level of analysis; it is lower when only closely related species are compared. For example, slopes are often 0.2-0.5 among species within a genus, but slopes average about 0.66 for families within orders. Evolutionary mechanisms that assume varying patterns of selection acting on brain and body size have been advanced to explain the change in slopes. Although real changes in slope with taxonomic level may occur, we show that the changes observed in real data sets can arise as a statistical artifact, owing to the nature of error variation in the comparative data and to the methods used to estimate slopes. The result holds for regression, major-axis, and reduced-major-axis estimators of the slope. Before applying evolutionary interpretations to observed changes in slope with taxonomic levels, investigators should adjust for the effects of error on estimated slopes.